Metal fluoride crystals are used in various applications, for example, as optical elements in lithography equipment used in semiconductor processing. Semiconductor lithography equipment operating at 193 nm wavelengths commonly uses fused silica optical elements, but a limitation of fused silica is that it is damaged by high fluence at 193 nm. The next generation of semiconductor lithography is expected to use 157 nm wavelength illumination, and fused silica is quite opaque to 157 nm wavelength illumination.
Calcium fluoride is a candidate material for use in optical elements in 193 nm and 157 nm lithography equipment. However, current crystal growth and annealing processes lead to high residual stress in large calcium fluoride crystals, thus limiting the ability to produce calcium fluoride crystals suitable for optical lithography. High residual stresses in a crystal can cause it to exhibit a spatially varying index of refraction, which will be referred to herein as homogeneity or inhomogeneity (depending on use in context). Stresses can similarly cause the index of refraction to vary with polarization, an effect known as stress induced birefringence. When used in an optical system, these stress induced effects can lead to wavefront errors, image degradation and birefringence, all of which are detrimental to the effectiveness of an optical system using calcium fluoride optical elements.
Calcium fluoride crystal growth is typically done using the Bridgman or Bridgman-Stockbarger method. The process starts by placing a crucible of raw material into the hot zone of a furnace where the raw material is melted. The crucible is then slowly translated out of the hot zone and through a temperature gradient region where crystal growth occurs as the molten raw material cools below its melting point. Since crystal growth is not an isothermal process, thermal stress is induced in the growing crystal. The nature of the growth process makes it very difficult to achieve a good homogeneity and a low stress birefringence in the crystal.
As a result of the stresses introduced during crystal growth, “post-annealing” or “secondary annealing” is typically used to reduce the inhomogeneity and birefringence in CaF2 crystals. However, a separate post-annealing process presents two major disadvantages. First, an extra furnace and process is required for post-annealing. This prolongs the total CaF2 manufacturing time and increases costs. Second, post-annealing adds another opportunity to damage the crystal either due to contamination or any process failure. Consequently, efforts have been directed to the development of an “in situ” annealing/cooling procedure just after the growth so that a high-quality crystal with improved homogeneity and birefringence can be obtained in one furnace by one process. However, despite these efforts there is a need for further improvements in methods of producing crystals that minimize index inhomogeneity and birefringence in calcium fluoride and other metal fluoride crystals.